This invention pertains to a method for producing thermally stable dielectric coatings comprising a first coating having a thickness of 1.25 to 2.25 .mu.m produced from a hydrogen silsesquioxane resin and a second coating having a thickness of at least 100 nm comprised of silicon dioxide (SiO.sub.2). It has been found that when a silicon dioxide coating having a thickness of at least 100 nm is applied over the hydrogen silsesquioxane resin based coating that the hydrogen silsesquioxane coatings resist cracking in subsequent thermal processing thus producing thermally stable dielectric coatings.
A method for formation of multi-layer coatings of limited thicknesses for interlevel dielectrics and passivation on semiconductor devices is known in the art. Under the current application method, as described in U.S. Pat. No. 5,145,723, a hydrogen silsesquioxane resin is spun onto the semiconductor device, and any solvent is removed to produce a hydrogen silsesquioxane film on the device. The hydrogen silsesquioxane film is heated by placing the semiconductor device on hot plates (150.degree. C. to 350.degree. C.) to soften and flow the hydrogen silsesquioxane resin and finally the hydrogen silsesquioxane film is cured by heating in an oven at about 400.degree. C. to 450.degree. C. for 1 hour. However, the method is effective for producing crack-free coatings only with thin layers (&lt;1.2 .mu.m) of hydrogen silsesquioxane resin ceramic. This thin layer does not adequately cover the metal layer and therefore to produce the interlevel dielectric it is required to apply a thick SiO.sub.2 layer over the hydrogen silsesquioxane ceramic.
Another method for producing multi-layer coatings on integrated circuits from hydrogen silsesquioxane resin is described in U.S. Pat. No. 4,756,977 to Haluska et al. According to '977 a first layer is produced by applying a solvent solution of hydrogen silsesquioxane resin to the device, the solvent is removed and the coating is ceramified by heating to a temperature of between 150.degree. C. and 1000.degree. C. The hydrogen silsesquioxane resin based coatings produced by the method described in '977 have a thickness of approximately 3,000 to 5,000 angstroms (0.3 to 0.5 .mu.m). A passivating coating is applied over the hydrogen silsesquioxane resin based coating. The passivating coating may be a Si, SiN, SiC or SiCN containing coating.
It is desirable to have coatings produced from hydrogen silsesquioxane thicker to adequately cover the metallization. However, when thick coatings are produced using the current processing methods, they contain undesirable cracks.
Other methods for curing hydrogen silsesquioxane resin are known in the art, however, these methods do not teach how to form thick coatings (1.25 .mu.m or greater). For example, U.S. Pat. Nos. 5,380,567, 5,370,904 and 5,370,903 describe curing of hydrogen silsesquioxane resin in an inert atmosphere. U.S. Pat. No. 5,380,567 to Haluska et al. discloses the cure of hydrogen silsesquioxane resin in an inert atmosphere at temperatures of 500.degree. C. to 1000.degree. C. (coating thickness 0.2 .mu.m). U.S. Pat. No. 5,370,904 to Mine et al. discloses a method for the formation of thick silicon oxide films wherein the method comprises forming a hydrogen silsesquioxane resin film on the surface and thereafter heating the film in an inert atmosphere at a temperature of from 250.degree. C. to 500.degree. C. until the content of the SiH in the silicon oxide product has reached .ltoreq.80% of the content of SiH in the hydrogen silsesquioxane (coating thicknesses 1.0-1.23 .mu.m). U.S. Pat. No. 5,370,903 to Mine et al. discloses a method for the formation of thick silicon oxide films wherein the method comprises forming a hydrogen silsesquioxane resin film on the surface and thereafter heating the film in a mixed gas atmosphere (.ltoreq.20% vol O.sub.2) at a temperature of from 250.degree. C. to 500.degree. C. until the content of the SiH in the silicon oxide product has reached .ltoreq.80% of the content of SiH in the hydrogen silsesquioxane (coating thicknesses 1.02-1.10 .mu.m).
Additionally, U.S. Pat. No. 5,059,448 to Chandra et al. discloses the use of rapid thermal processing to produce coatings of 1 .mu.m or less (0.13 to 0.945 .mu.m). In '448 the hydrogen silsesquioxane resin film is exposed to a high intensity radiation to quickly heat the film at a temperature of 50.degree. C. to 1000.degree. C.
Thicker coatings have been produced by adding fillers to hydrogen silsesquioxane resin. However, because of the presence of the filler and the effect of the filler on the properties of the coating, these coatings are not necessarily suitable as interlevel dielectrics. For Example, U.S. Pat. No. 5,458,912 to Camilletti et al. discloses a method for forming tamper-proof coatings on electronic devices by applying to the device a coating comprising a silica precursor and a filler and thereafter heating at a temperature sufficient to convert the silica precursor to a silica containing ceramic matrix. The coatings produced have thicknesses of 20 to 48 .mu.m.
It has now been found that when the cure conditions (time, temperature and environment) are controlled, a crack-free insoluble coating having a thickness of greater than 1.25 .mu.m can be produced from hydrogen silsesquioxane resins. It has also been found that when a layer of silicon dioxide having a thickness of at least nm is applied over the hydrogen silsesquioxane resin based coating, the coatings are stable (remain crack-free) when exposed to subsequent thermal processing.